1. Field of the Invention
The present invention relates to a photosensitive resin composition. More particularly, the present invention relates to a photosensitive resin composition having an excellent photosensitivity, water resistance, water developing property, smoothness, resolving power, and solvent resistance, and valuable for the production of screen printing plates, second originals, and television fluorescent screens
2. Description of the Related Art
A photosensitive composition comprising saponified polyvinyl acetate and a styrylpyridinium salt compound or styrylquinolinium salt compound is known. The photoreaction is photocrosslinking by photodimerization of the styrylpyridinium salt compound or styrylquinolinium salt compound grafted to the saponified polyvinyl acetate. Accordingly, the amount of the hydroxyl group contained in the saponified polyvinyl acetate in the photosensitive layer is not changed by irradiation. Therefore, the water resistance of the portion cured by irradiation is not improved, and the cured portion is extremely swollen during the water development and a high resolving power cannot be obtained.
The following two methods have been tried for eliminating this defect
(A) Method using a protecting colloid formed of saponified polyvinyl acetate or a derivative thereof
When a photosensitive resin composition comprising a reaction product of saponified polyvinyl acetate with a styrylpyridinium salt compound or styrylquinolinium salt compound is used for formation of an image, in order to enhance the water resistance of an image formed through the light exposure and water development steps, a method is used in which an aqueous emulsion of a polyvinyl acetate resin comprising saponfied polyvinyl acetate as the protective colloid is added to the photosensitive resin composition. In this method, in order to further enhance the water resistance of the image, the content of the protective colloid of the saponified polyvinyl acetate must be controlled to a low level such as 2 to 10% based on the weight of the resin solids of the emulsion.
An improvement of this method using the protective colloid is proposed in Japanese Unexamined Patent Publication (Kokai) No. 59-102232. According to this improved method, a photosensitive unit having a photocrosslinking property is bonded to the main chain of saponified polyvinyl acetate used as the protective colloid so as to further improve the water resistance of the formed image. In this method, it is preferred that the amount of the photosensitive saponified polyvinyl acetate derivative used as the protective colloid be controlled to less than 10%. It is admitted that if this amount exceeds 10%, a desired water resistance cannot be maintained. Furthermore, in the case of a polyvinyl acetate resin-containing emulsion comprising less than 10% of the above-mentioned photosensitive saponified polyvinyl acetate as the protective colloid, the contribution to improvement of the water resistance is not substantially different from the contribution attained when an emulsion comprises an unmodified saponified polyvinyl acetate derivative as the protective colloid. When a polyvinyl acetate resin-containing emulsion comprising less than 10% of the above-mentioned photosensitive saponified polyvinyl acetate derivative as the protective colloid is used, the water resistance of the obtained image is considerably good, but there still remains another problem in that the solvent resistance of the image is very poor. This degradation of the solvent resistance may be prevented by increasing the amount of the reaction product of saponified polyvinyl acetate with the styrylpyridinium salt compound or styrylquinolinium salt compound relative to the polyvinyl acetate resin-containing emulsion, but in this case, reduction of the water resistance of the formed image is caused. Namely, in the method in which the above-mentioned photosensitive saponified polyvinyl acetate is used as the protective colloid for a hydrophobic resin emulsion, it is impossible to simultaneously impart satisfactory improvements of the water resistance and solvent resistance to the formed image.
(B) Method in which saponified polyvinyl acetate or a derivative thereof is rendered hydrophobic
Use of a photosensitive resin formed by partially acetalizing the --OH group of saponified polyvinyl acetate by an aldehyde compound to render the saponified polyvinyl acetate hydrophobic and bonding the above-mentioned photosensitive unit to the hydrophobic saponified polyvinyl acetate is effective for improving the water resistance of the formed image. However, if the hydrophobic degree is increased, gelation is often caused in the aqueous emulsion containing the obtained photosensitive resin during the storage. In order to prevent occurrence of this gelation, it is necessary to use a mixture of an organic solvent such as an alcohol and water as the medium of the emulsion, instead of water alone.
Under this background, it is eagerly desired in the art to develop a photosensitive resin containing water alone as the medium, which is capable of forming an image having an excellent water resistance and solvent resistance and having a high resolving power.